The present invention is directed to an acrylic polymer emulsion, which, when blended with an elastomeric material can be used as a coating for use on various products. One especially useful application is for coating a film or paper substrate for use as a sheet upon which images may be printed with ink jet printers. When applied to paper or film to produce print sheets, the coating provides a high quality, clear, glossy surface that freely receives water based inks such as are commonly used with ink jet printers. These properties make print sheets coated with the polymer especially useful as photographic quality papers and transparencies for use with ink jet printers. Another application for the polymer is for coating rubber articles such as gloves. When applied to such rubber articles, excellent mold release, donning and anti-blocking properties can be achieved.
Ink jet printing is a well-known and commonly used means of producing an image onto a substrate. Ink jet printers typically use one of two different types of ink, dye-based inks and pigment-based inks. With dye-based ink, the color of the ink is imparted by a dye that is soluble in a fluid carrier. The fluid carrier is typically a blend of water and glycol. Such dye-based inks are relatively inexpensive and easy to process, and are suitable for use in low cost applications in which long term durability is not a concern. In pigment-based inks, the color is imparted by particles that are dispersed, rather than dissolved, in a fluid carrier. Water-soluble fluids are typically used as the carrier for pigment-based ink for improved lightfastness.
Some of the most recent advances in ink jet printer technology have been in the area of photo-quality or near photo-quality printing. While printers and inks are able to achieve the high resolution necessary for photo-quality printing, many of the printing papers and other printing films (such papers and films collectively referred to in this specification as xe2x80x9cfacestocksxe2x80x9d) available to the consumer fail to permit the consumer to take full advantage of the printer""s capabilities. A first shortcoming is that many of the photo-quality facestocks that are capable of high-resolution photo-quality printing do not impart sufficient glossiness to match that of prints made on photographic paper by traditional photographic techniques. A second shortcoming is that many of the photo-quality facestocks with the desired glossiness are incapable of the high resolution desired for a photo-quality print. To the extent that some facestocks have been able to achieve both high gloss and high resolution, still further problems such as slow dry times are often realized. It is important that the ink applied to a facestock be able to dry fast to permit the handling of the print soon after printing, and to allow printed sheets to be stacked in a printer""s output tray without resulting in any smearing the printed images.
In order to meet the print facestock requirements for high gloss, good ink absorbency and fast print drying, prior art ink jet facestock technology often relies on the use of various coatings which are applied to a substrate. For many such coatings, multiple layers are required which tend to require elaborate, and therefore, expensive manufacturing techniques. Many coatings also require mechanical treatment such as the use of a polished casting drum in order to achieve the desired glossiness, a step that still further adds to the manufacturing cost. Moreover, such facestocks still tend to be slow drying, making them susceptible to smearing of the printed images. Improved facestock coatings that are fast drying, glossy and capable of high-resolution are desired for the production of facestocks used for photo-quality computer printing applications.
Rubber articles made from natural or synthetic rubber include surgical gloves, physician examining gloves, industrial work gloves, prophylactics, catheters, balloons, tubing, sheeting and the like. Some of these articles, and in particular gloves, require good donning properties, that is, the ability of the rubber article to be slid on and off skin surfaces without undue clinging or friction. Surgical gloves require good wet donning properties, that is, the ability to be slid over damp or wet skin surfaces. Physician examining gloves and industrial work gloves require good dry donning properties, that is, the ability to be slid over dry skin surfaces.
The comfort of such a glove, whether worn for surgical use, examination use, or industrial use, can generally be improved, especially if the glove is worn for a prolonged period of time, if the inside of the glove is absorbent. Furthermore, good absorbency properties in which the buildup of moisture within a glove is prevented can improve the wearer""s dexterity. However, while absorbency is desirable, the absorbency of a rubber article such as a rubber glove cannot be permitted to compromise the barrier properties required of such gloves.
Another desirable property for gloves and other rubber articles is that the outer surfaces permit such articles to be stacked against one another without sticking or xe2x80x9cblockingxe2x80x9d to one another. Of course, neither should the interior of a glove or other rubber article stick to itself as such blocking makes donning difficult.
It is also desirable that any coating that is to be applied to the mold side of the article impart good mold release properties. Without good mold release properties, the finished articles can be difficult to remove from the molds, especially if mechanical devices are used for stripping the finished articles from the molds. Without good mold release properties, a significant number of articles may be torn during the stripping step. Yet another benefit of a coating with good mold release properties is that it simplifies the cleaning of the mold. This is important, as a single mold should be capable of producing a large number of articles without having to be taken off the manufacturing line for elaborate cleaning procedures.
To achieve the desirable characteristics for gloves and other rubber articles, various coatings have been developed. Such coatings can be applied to the inner, outer, or both surfaces of the rubber article. One of the simplest coatings has been to apply a powder such as talc or cornstarch to the surfaces of the article. The use of such a powder improves both the donning and blocking characteristics of such gloves.
The conventional way of manufacturing rubber articles such as powdered gloves has been to dip a mold or former, having the shape of the article to be formed, into a coagulant slurry containing both powder and a coagulant such as calcium nitrate or calcium carbonate. After drying, the mold is immersed in a rubber emulsion for a time sufficient for the rubber to coagulate and form a coating of desired thickness. Water leaching is generally employed as the next step in order to remove rubber impurities. Once the leaching process has been completed, the rubber article is then cured and dipped into a starch slurry. The starch-coated surface is then dried, leaving a coating of powder on the surface of the glove. After cooling, the rubber article is stripped from the mold. For most rubber articles, the stripping step results in the glove being turned inside out. The mold is then cleaned and used again for making another article.
An important drawback to the use of powder-coated gloves and other articles is that for many applications, the powder can contaminate the environment in which the wearer of the article is working. For example, for certain surgical procedures, powder-coated gloves are unacceptable as such a powder coating can contaminate the surgical field. Similarly, for gloves worn in clean rooms such as are commonly found in the electronics industry, the powder coating of a rubber glove can cause undesirable contamination of the clean room. Therefore, coatings are desired which can achieve the desirable donning and blocking properties for rubber articles without the use of powders.
Methods and materials used for glove manufacture are described, for instance, in U.S. Pat. Nos. 3,411,982 and 3,286,011 to Kavalier et al., both incorporated herein by reference, xe2x80x9cPolyurethane Latexes for Coagulation Dipping,xe2x80x9d Sadowski et al., Elastomerics, August 1979, pp. 17-20, incorporated herein by reference, and xe2x80x9cDipping with Natural Rubber Latex,xe2x80x9d Pendler et al., Natural Rubber Technical Bulletin, 1980, also incorporated herein by reference.
One way to eliminate the traditional powder coatings for gloves has been through the use of one of various surface treatment methods for the finished article. One such surface treatment method requires the halogenation of the surface of the article. Chlorination is the most commonly used of such methods. However, while effective, such treatments are often expensive. Moreover, they often have adverse affects on the shelf life of the rubber articles formed. It would be desirable to provide a rubber article with a powder-free donning surface without resorting to the expensive and article-deteriorating practices that are now commonly used. Such a process could substantially reduce the cost of manufacture and maximize the shelf life of the rubber article.
Other methods of making powder-free rubber articles such as gloves have included the use of coatings with silicon-based chemicals. However for the clean rooms employed in electronic chip manufacturing activities, the inclusion of silicon in the coatings is unacceptable as it can cause silicon contamination of delicate electronic components. Therefore, a coating for gloves and other rubber articles that is both powder-free and silicon-free while providing good donning, mold release and anti-blocking properties is desired.
The present invention sets forth a polymer emulsion useful for forming a coating a number of different products. The polymer emulsion is produced as the reaction product of a monomer mixture comprising an alkyl (meth)acrylate, a quaternary amine (meth)acrylate, a hydroxyalkyl (meth)acrylate, an N-vinyl lactam, an ethylenically unsaturated carboxylic acid, and a fluorinated (meth)acrylate. Additionally, a hard monomer and/or an ethoxylated (meth)acrylate monomer may be provided. The polymer emulsion is produced under vigorous mixing and preferably in the presence of a plurality of surfactants to further ensure good mixing. In the preferred embodiment, the polymer emulsion is made by a sequential polymerization reaction of two different monomer mixtures. For such a sequential polymerization reaction, the first monomer mixture is first allowed to partially react before the second monomer mixture is added to the reactor. Improved properties for the resulting polymer coating result when the second monomer mixture has a lower amount of ethylenically unsaturated carboxylic acid than the first monomer mixture. In particular, when such a sequential polymerization reaction is used, the resulting polymer emulsion has a lower viscosity at higher solids concentration than would otherwise be achieved if a single monomer mixture were reacted. By having a lower viscosity at a higher solids content, the polymer emulsion is far easier to handle.
The polymer emulsions of the present invention are useful in forming coatings for a variety of materials. As one example, the coatings can be applied to a facestock such as paper or vinyl in order to make a coated facestock for use in ink jet printing applications. Such coated facestocks exhibit excellent clarity and gloss characteristics, are fast drying and permit the printing of high-resolution photo-quality images using ink jet printers. Furthermore, even though such coated facestocks exhibit high gloss, they are still highly flexible and resist cracking and separation from the substrate. Moreover, the coated facestocks are easily fed through ink jet printers without causing jamming or other difficulties for the printer. Preferably, the coating is formed by diluting the polymer emulsion with an elastomeric material such as a nitrile rubber latex to a concentration of about 95% polymer emulsion. The coating is applied to a facestock at a dry coat weight of between about 10 and about 50 grams per square meter.
The polymer emulsions are additionally useful in forming coatings for rubber articles. For molded rubber articles, such coatings exhibit excellent mold release properties, even when mixed with a high concentration of coagulants such as calcium nitrate which are often used in the forming of nitrile rubber latex articles. In addition to outstanding mold release properties, the coatings provide excellent anti-blocking properties when applied to the outside surface of the rubber article. When applied to the inside surface of a rubber article such as a rubber glove, the coatings still further provide excellent donning properties, for both wet donning and dry donning applications. Moreover, the absorbency imparted to an article such as a glove whose inside surface is coated with the polymer coating of the present invention make the glove more comfortable to wear for extended periods of time. Preferably, such a polymer coating is formed by diluting the polymer emulsion with an elastomeric material such as a nitrile rubber latex. Preferably, the polymer emulsion is diluted with nitrile rubber latex to a concentration of about 25% by weight on a dry basis. The resulting emulsion of polymer and nitrile rubber latex is then diluted with water to a concentration of between about 2 and 8% solids to form a coating for a nitrile rubber article. Articles coated with the polymer coating of the present invention are strong, flexible and exhibit very high stretch without appreciable cracking or separation of the coating from the article. Such coatings can be applied to natural latex articles as well as to nitrile rubber latex articles.
The polymer emulsions of the present invention are acrylic emulsion copolymers that are the reaction product of a mixture of monomers. In this specification, the term xe2x80x9cmonomerxe2x80x9d is meant in a broad sense to encompass monomers and oligomers as would be used in building the desired copolymers. The monomers generally include one or more alkyl (meth)acrylate monomers. As used in this specification, the term (meth)acrylate is intended to refer to either an acrylate or a methacrylate. For example, the term alkyl (meth)acrylates is meant to refer to the group of chemicals that includes both alkyl acrylates and alkyl methacrylates.
The monomer mixture also includes one or more quaternary amine (meth)acrylate monomers, one or more hydroxyalkyl (meth)acrylate monomers, one or more N-vinyl lactam monomers, one or more ethylenically unsaturated carboxylic acid monomers and one or more fluorinated (meth)acrylate monomers. In the preferred embodiment, the monomer mixture further includes one or more hard monomers and one or more ethoxylated (meth)acrylate monomers.
The alkyl (meth)acrylates and hard monomers together generally provide the durability and glossiness to the polymer emulsions. Preferred alkyl (meth)acrylates are those having from 1 to 10 total carbon atoms. Examples of suitable alkyl (meth)acrylates monomers include methyl acrylate, ethyl acrylate, butyl acrylate, ethylhexyl acrylate, isooctyl acrylate, methyl methacrylate and isobutyl methacrylate. As used herein, the term xe2x80x9chard monomerxe2x80x9d is meant to include monomers including alkyl (meth)acrylate and styrene monomers having a glass transition temperature (Tg) greater than room temperature or about 25xc2x0 C. Examples of suitable hard monomers include alkyl (meth)acrylates monomers with a Tg greater than about 25xc2x0 C., such as methyl methacrylate and isobutyl methacrylate, and styrene and styrenic monomers.
While the use of at least one hard monomer is desired in the preferred embodiment for durability and to prevent tackiness of the resulting coatings, at least a portion of the one or more alkyl (meth)acrylates selected should also have a Tg of less than 25xc2x0 C. in order to prevent the coatings from being too brittle. Most preferably, a combination of alkyl (meth)acrylates and hard monomers should be selected to achieve the desired Tg for the coating. In the preferred embodiment a combination of methyl acrylate, butyl acrylate and methyl methacrylate is provided. The one or more alkyl (meth)acrylates, are preferably provided in a total amount between about 60 and 80% by weight of the total reactants. The hard monomers are preferably provided in a total amount between about 5 and 15% by weight of the total reactants. It should be apparent to one of ordinary skill in the art that some alkyl (meth)acrylates such as methyl methacrylate or isobutyl methacrylate fall within the definition of a hard monomer as well as the definition for an alkyl(meth)acrylate. Therefore, to the extent the preferred embodiment calls for amounts of both types of monomers, the inclusion of an alkyl (meth)acrylate with a Tg greater than 25xc2x0 C. will satisfy both requirements.
Quaternary amine (meth)acrylates are cationic monomers that, when combined with the other monomers are considered to provide improved water absorbency properties to the coatings. The preferred quaternary amine (meth)acrylate is a product knows as Ageflex FA11Q80MC which is an 80% aqueous solution of dimethylaminoethyl acrylate methyl chloride quaternary available from CPS Chemical Co. of Old Bridge, N.J. The quaternary amine (meth)acrylate is preferably provided in an amount between about 5 and 15% by weight of the total monomers.
The hydroxyalkyl (meth)acrylate monomers are also generally considered as providing improved water absorbency characteristics to the coatings of the present invention. They also provide pigment-bonding properties that are especially useful for film coatings for printing applications using pigment-based inks. The preferred hydroxyalkyl (meth)acrylate monomer is 2-hydroxyethyl methacrylate (HEMA) such as that available from Mitsubishi Rayon Co., Ltd. of Tokyo, Japan. The hydroxyalkyl (meth)acrylate monomer is preferably provided in an amount between about 5 and 35% by weight of the total monomers.
The inclusion of one or more N-vinyl lactam monomers in the monomer mixture, together with the hydroxyalkyl (meth)acrylate monomers, is generally considered to help fix the dye to the coating when the coating is used for making coated facestocks for use with printers. The preferred N-vinyl lactam monomers are N-vinylcaprolactam monomers and the more preferred are vinyl pyrrolidones. The most preferred is N-vinylpyrrolidone. Such a product is available from ISP Technologies Inc. of Wayne, N. J. under the trade name V-PYROL/RC. Preferably, the N-vinylcaprolactam is provided in an amount between about 5 and 15% by weight of the total monomers.
The one or more ethylenically unsaturated carboxylic acids, when combined with the hydroxyalkyl (meth)acrylate, also helps to improve the water absorbency characteristics of the coatings. The preferred ethylenically unsaturated carboxylic acids are alkyl (meth)acrylic acids, carboxyethyl acrylates and itaconic acids. The most preferred ethylenically unsaturated carboxylic acids are acrylic acid and methacrylic acid. The preferred range for the ethylenically unsaturated carboxylic acids is between about 4 and 15% by weight of the total monomers. If less than about 4% of acid is provided, the resulting polymer coating tends to have too slow of a dry speed when coated to a film for use in an ink jet print film application.
The one or more fluorinated (meth)acrylate monomers tend to bloom to the surface of the coating once cured and are generally considered to prevent the coated articles from blocking. When the coating is used for coating facestock used for printing, the fluorinated (meth)acrylate monomers also helps the printer to better grip the facestock for feeding it into the printer. Surprisingly, this benefit is provided without adversely affecting the desirable glossiness of the coated facestock. When a coating with fluorinated (meth)acrylate monomers is used for producing coated rubber articles such as gloves, the resulting gloves have improved tactile feel. The preferred fluorinated (meth)acrylate monomer is trifluoroethyl methacrylate. Trifluoroethyl methacrylate is available from Elf Atochem North America of Philadelphia, Pa. under the trade name MATRIFE. Because the fluorinated (meth)acrylate monomers tend to bloom to the surface, very small amounts are effective at achieving the desired properties. Therefore, any positive amount of fluorinated (meth)acrylate monomer is preferred. Most preferably the fluorinated (meth)acrylate monomers are provided in an amount between about 0.1 and 5% by weight of the total monomers.
In one preferred embodiment, one or more ethoxylated (meth)acrylate monomers are also provided to the monomer mixture to act as an active surfactant to both improve the reaction conditions during polymerization and improve the properties of the coatings by reducing the surface tension. The preferred ethoxylated (meth)acrylate monomers are ethoxylated hydroxyalkyl (meth)acrylates with an average of between 5 and 20 ethylene oxide units. The most preferred are ethoxylated hydroxyalkyl (meth)acrylates with an average of between 10 and 15 ethylene oxide units. An ethoxylated hydroxyethylmethacrylate with an average of 15 ethylene oxide units is available in an 88% by weight solution with 12% by weight propyleneglycol monomethylether from Nippon Nyukazai Co., Ltd. of Tokyo, Japan under the product designation MA-150MF. An ethoxylated hydroxyethylmethacrylate with an average of 10 ethylene oxide units is available from Nippon Nyukazai Co., Ltd. under the product designation MA-100A. The ethoxylated (meth)acrylate is preferably provided in a positive amount between about 0 and 5% by weight of the total monomers, most preferably between about 0.1 and 5% by weight.
The coatings made from the polymer emulsions provided by the reaction of the individual monomer components set forth above need to be durable and flexible without being tacky on the one hand, or brittle on the other hand. As the individual monomers that make up the monomer mixture, with the exception of the low Tg alkyl (meth)acrylates, tend to all have glass transition temperatures above room temperature, it is important to include at least some monomers with low Tg. Otherwise, if the concentration of monomers with high Tg is too high, and an insufficient amount of monomers with low Tg are included, the resulting coatings will tend to be too brittle. Therefore, the upper limits for the amount of each of the individual monomers is generally dictated by Tg limitations.
The reaction of the monomers is carried out by known methods. In the preferred embodiment, a heated reactor is initially charged with deionized water and one or more surfactants. Because a number of monomers with different physical properties are being reacted, in the preferred embodiment, a number of different anionic and nonionic surfactants are provided, each selected for improving the miscibility of a particular monomer or group of monomers. The initial reactor charge is subjected to a nitrogen purge to remove oxygen, thereby reducing the extent of undesirable oxidation reactions. Because thorough agitation is critical to the reaction, vigorous agitation is started immediately and is continued throughout the reaction. A small amount of defoaming agent is also added at this time to reduce foaming of the reactants. A suitable defoaming agent is available from Drew Industrial Division of Ashland Chemical Company of Boonton, N.J. under the trade name Drewplus L-191. An amount of activator is also added. The most preferred activator is the sodium salt of ethylenediaminetetraacetic acid, iron(III), or Fe-EDTA, which further acts as an oxygen scavenger. Other preferred activators include ascorbic acid and sodium metabisulfite.
Prior to their introduction into the reactor, the monomers, except for the quaternary amine (meth)acrylate, are separately mixed with one another. The pH of the monomer mixture is measured and a base such as ammonia is added under good agitation to adjust the pH to within a slightly basic range. Preferably, a 16% ammonia solution is used to adjust the pH to within the range of about 7.5 to 8. The quaternary amine (meth)acrylate monomer is then added and the pH is again measured. At this point, the mixture tends to be slightly cloudy. Deionized water is added until the dispersion is clear under vigorous agitation. It is important that the mixture be clear as any cloudiness in the monomer mixtures can ultimately result in cloudiness in the coatings produced. It is also important to recognize that the mixtures should be maintained under constant agitation as without such agitation, the dispersions will quickly separate into two layers. However, if such separation occurs, the desired dispersion can again be achieved under vigorous agitation.
Once the reactor reaches a temperature of about 45xc2x0 C., an initiator is introduced. Suitable initiators are water-soluble initiators such as azo bis(4-cyanovaleric acid) from Wako Pure Chemical Industries, Ltd. of Osaka, Japan, or tertiary-butyl hydroperoxide. Tertiary-butyl hydroperoxide is presently the most preferred. At this time, the nitrogen purge to the reactor is stopped and the monomer mixture is slowly added to the reactor along with an accelerator solution. A preferred accelerator is an aqueous solution of the sodium salt of hydroxymethanesulfinic acid. Such a product is available from Henkel of America, Inc. It is also important to recognize that the monomer mixture should be separately stirred during its addition to the reactor to prevent the individual ingredients of the monomer mixture from separating.
Upon the addition of the monomer mixture, the temperature of the reactor is allowed to rise due to the exothermic polymerization reaction. The reaction temperature is preferably maintained at between about 45xc2x0 C. and 55xc2x0 C., and most preferably between about 48xc2x0 C. and 53xc2x0 C. Once the monomer mixture has been completely added, the addition of the accelerator solution is continued for another about 40 to 60 minutes.
In order to complete the reaction, as a final cook-off, an additional amount of initiator is added to the reactor after the monomer mixture has been completely added to the reactor. The addition of the accelerator solution continues during this cook-off period. After the accelerator solution has been added, the mixture is kept warm for an additional about 30 minutes to complete the reaction. The reactor is then cooled. When the reactor temperature has fallen to about 35xc2x0 C., a small amount of biocide is added along with deionized water to adjust the total solids content. At this time, the pH is also adjusted to within the range of 7.0 to 8.5 by the addition of a suitable base such as ammonia. For ease of handling, and also to simplify mixing, the polymer emulsion is preferably produced as an emulsion with about 35% solids by weight. The reaction products are then filtered to remove any grit.
In the preferred embodiment, rather than preparing the polymer emulsion as the reaction product of a single monomer mixture, two separate monomer mixtures are provided with different properties. Preferably the second monomer mixture includes a lower concentration of ethylenically unsaturated carboxylic acid monomers than the first monomer mixture. According to this embodiment, the first monomer mixture is first added to the reactor and at least partially reacted. The second monomer mixture is then slowly introduced and reacted. By using such a procedure, a procedure referred to herein as a xe2x80x9csequential polymerizationxe2x80x9d procedure, and by carefully selecting the properties of the first and second monomer mixtures, the resulting emulsion can be produced with high solids content at relatively low viscosity. Such sequentially polymerized emulsions also produce coatings with improved properties such as faster dry times for ink jet printer papers and films coated with the polymer coating. In contrast, if a single feed is used, a higher viscosity generally results, making handling of the resulting emulsion somewhat more difficult. Ink jet printer papers and films coated with a polymer coating produced by a single feed polymerization reaction tend to have slower dry speeds. Without being bound by theory, applicant believes that the sequential polymerization procedure causes the polymer particles to form as a core of a first polymer formed by the reaction products of the first monomer mixture with a shell surrounding the core, the shell being formed by the reaction products of the second monomer mixture.
Once the polymer emulsion is prepared, it can then be applied to an appropriate substrate by known methods. Preferably, the a coating material is produced by diluting the polymer emulsion with an elastomeric material such as a rubber emulsion, most preferably a nitrile rubber latex. When the polymer emulsion is to be used for coating a facestock for use as a coated facestock for printer applications, the polymer emulsion is preferably diluted with a nitrile rubber latex to a concentration of about 5% by weight on a solid basis. Most preferably, the nitrile rubber latex is provided as an aqueous emulsion at about 48% by weight solids. The preferred nitrile rubber latex is a carboxylated butadiene-acrylonitrile rubber latex. One such carboxylated butadiene-acrylonitrile rubber latex is available from Synthomer Ltd. of Harlow, Great Britain under the trade name Synthomer 6000. In the preferred embodiment, the nitrile rubber latex is first diluted to a concentration of about 35% by weight solids and then added to the polymer emulsion.
The polymer coating material is applied to a facestock at a preferred dry coat weight of at least about 10 grams per square meter (gsm). Preferably, less than about 50 gsm of coating material is provided. At coat weights of less than about 10 gsm, it can be difficult to obtain a uniform finish on the paper or film. Thinner coat weights will also result in papers and films that take longer to dry once printed. While coat weights greater than about 50 gsm are possible with good properties including good flexibility and extremely rapid drying properties, such coat weights can tend to be expensive, and therefore, are not generally preferred.
As already mentioned, the coating can be applied to any number of different facestocks including substrates such as paper, card stock, cardboard and vinyl, polyester, polyethylene and polypropylene films. An important benefit of the polymer coating of the present invention is that it adheres well to virtually any substrate. Consequently, the choice of substrates is not of critical importance in practicing the invention. Preferred coat weights for paper, card stock and cardboard are 18 to 24 gsm. Preferred coat weights for vinyl, polyester, polyethylene and polypropylene films are in the range of 20 to 30 gsm.
As stated, in the preferred embodiment, a facestock coated with the polymer of the present invention has high gloss characteristics. Such high gloss is achieved without any mechanical treatment methods such as those commonly employed in the production of prior art high-gloss coatings. However, for some applications, a lower gloss finish may be desirable. In order to achieve a matte finish, up to about 20% by weight of particles, preferably inert particles such as silicate crystals having an average particle size of 5-10 microns, may be added to the coating before it is applied to the substrate. Of course, any degree of gloss between a dull, matte finish and a high gloss finish can be achieved by varying the amount of particles added.
While coat weights of at least 15 gsm are generally desired for photographic quality printer paper, it has surprisingly been discovered that at low coat weights in the range of about 10 to 18 gsm, a coated film produced from a conventional release liner coated with the polymer of the present invention has desirable image transfer properties results. Such an image transfer film is able to produce an image with high resolution. In order to transfer the image, the printed films is pressed against the substrate to which the image is to be transferred while the ink is still wet, allowed to dry, and the release liner is carefully peeled away. The image is transferred cleanly with a clean break between the printed image and the surrounding coating. Such a clean break is desirable in that the transferred image does not include a xe2x80x9cshadowxe2x80x9d of surrounding coating as typically results with image transfer products. Such results are also achieved without the need for supplemental adhesives as the printed image is inherently tacky when wet. Possible substrates to which such an image may be transferred include any facestock such as paper, card stock, cardboard, vinyl, polyester, polyethylene, polypropylene, and even substrates such as glass, ceramic and cloth.
If desired, an image may first be printed to an image transfer film and allowed to dry for later transfer to some other substrate. When it is desired to transfer the image, the image is first dampened with water such as by spraying the printed side of the transfer film and then the transfer film is pressed against the substrate as set forth above. One difference in using the transfer film in this way is that the film is also transferred to the substrate without the clean break achieved when the image is transferred immediately after printing. However, such a method is useful in producing a transfer film with remoistenable, water-activatable adhesive characteristics. Such characteristics can be useful for wallpaper, postage stamps, and a number of other different applications.
Yet another desirable property of the image transfer films of the present invention is that the transferred images, though not waterproof, are water resistant. Once the image has been transferred to a substrate, the removal of the image requires both water and a mechanical shear force for removal. Therefore, such image transfer films can be useful in producing attractive container labels for jars and bottles of food and other products. Such labels will tend to adhere well to the container, but can easily be removed when desired.
As discussed, the polymer emulsion is also useful in producing a coating for rubber articles such as gloves. The same absorbency properties that make the polymer coating useful for making photographic quality printer paper also make the coating useful as a donning coating for items such as rubber examination gloves. The coating is additionally useful as a release coating for rubber articles made by molding techniques. Whether it is used as a donning coating or release coating, the polymer coating of the present invention provides good anti-blocking properties, and the coated rubber articles are less likely to stick to themselves, or to each other when stacked.
When used as a coating for rubber articles, the polymer emulsion is preferably blended with an elastomeric material such as a rubber latex emulsion, most preferably a nitrile rubber latex. The preferred nitrile rubber latex for coating rubber articles is a carboxylated butadiene-acrylonitrile latex product available under the trade name Synthomer 6000, as set forth above. While alone, the cured polymer tends to be fairly brittle, when blended with nitrile rubber latex, the elastic properties of the coating are greatly enhanced with elasticity of 1000% or higher. Furthermore, the inclusion of nitrile rubber latex imparts desirable water resistant properties to the coating, and results in a strong bond between the rubber article and the coating. This makes the coated rubber article less likely to crack after stretching.
In one preferred embodiment, in addition to being diluted with a nitrile rubber latex, the polymer emulsion is blended with an acrylate, silicone, styrene and urethane polymer emulsion. Such an emulsion polymer is disclosed in U.S. patent application Ser. No. 09/215,578, which is incorporated herein by reference. The inclusion of an acrylate, silicone, styrene and urethane polymer emulsion with the polymer coating of the present invention can greatly improve the elasticity of coatings made from the acrylate, silicone, styrene and urethane polymer emulsion alone.
When used as a release coating, the polymers of the present invention have the further desirable property of being able to tolerate high concentrations of the metal salts used as coagulants for the production of rubber articles. This simplifies the manufacturing steps in making rubber articles. In particular, a blend of a nitrile rubber emulsion and a polymer emulsion of the present invention can be combined with a coagulant such as calcium nitrate and used as a preliminary dipping emulsion to replace an emulsion of powder and coagulant such as is used in a conventional assembly line for the manufacture of rubber articles. Surprisingly, it has been discovered that the blending of the polymer emulsion of the present invention with nitrile rubber latex permits the nitrile rubber latex to tolerate high levels of coagulant without coagulating as would occur if the polymer emulsion of the present invention were not included. According to the invention, calcium nitrate levels as high as 35% can be tolerated at temperatures up to 60xc2x0 C. For the preferred embodiments of the invention in which an ethoxylated (meth)acrylate monomer is included in the monomer mixture, calcium nitrate levels nearing the maximum solubility of calcium nitrate in water, about 43.5% at 60xc2x0 C., can be tolerated. Such levels are useful for making thick rubber articles such as industrial gloves where high levels of coagulant are required.
In the preferred embodiment, a mold or former such as a glove former is dipped into such a blend to form a first coating on the former. This first coating is then permitted to briefly dry. Once it has dried, it is then dipped into a conventional rubber emulsion and allowed to set. The formed rubber article is then subjected to a leaching step to remove impurities. The rubber article is then dipped into a second blend of nitrile rubber latex and the coating polymers of the present invention and cured. Once cured, the article is stripped from the former, the former is cleaned and the process is repeated. It should be noted that while the first coating will not coagulate prematurely, during the cure step the coating coagulates and cures to provide a coating that is securely bonded to the rubber article.
Since rubber articles are generally turned inside out during the stripping step, the outer coating applied during production is often the inner surface of the finished article. For example, for gloves, the coating applied as the outer coating during manufacture will typically be the donning coating of the finished glove. For those articles that are turned inside out after formation, the first coating which initially serves as a release coating for the former, further acts as anti-block coating for the outer surface of the formed rubber article. The second coating in turn acts as the donning coating for the rubber article.
While the invention has been described with specific reference to its use in forming gloves by multiple dipping steps, the term xe2x80x9ccoated rubber articlexe2x80x9d is meant to include a rubber article with a coating adjacent at least one side of the rubber article regardless of how the rubber article was manufactured. Specifically, the term includes those articles in which a rubber article is first formed and then coated with the polymer coating as well as those articles for which the polymer coating is first formed and then the rubber article is molded over the polymer coating. Furthermore, while specific reference is made to nitrile rubber latex articles, the invention is also useful for producing coatings for other rubber articles such as natural rubber latex articles.